Variable attenuators are important components in fiber optical test and measurement instrumentation and in fiber optical communication systems. The requirements of optical attenuators for use in such systems are that they should vary the intensity of the light transmitted without appreciably altering the spatial, temporal, spectral or polarization distribution of the light beam, and that they should be polarization insensitive.
Many types of variable attenuators have been described in the prior art. Mechanically variable attenuators, such as those using moveable or rotatable graded neutral density filters, or movable absorbing sections inserted into the optical path are bulky, complex and slow, and so are unsuitable for fast response optical communication use.
Electronically controlled fiber optical variable attenuators have been described in a number of prior art patents. In U.S. Pat. No. 6,055,104 there is described a variable attenuator which uses a polarizing beam splitter and a polarization rotator, which could be a Faraday rotator, a magneto-optical effect crystal, or a liquid crystal. Variable attenuators using similar schemes of polarization rotation and control elements are described in U.S. Pat. Nos. 5,999,305, 5,978,135, 5,973,821, 5,963,291, 5,867,300 and 5,727,109, and in the numerous previous references cited in these Patent documents.
All of these variable attenuators are generally complex in construction, in that they usually involve at least three optical elements, a polarizing element, an electro-optical polarization control element and an analyzing element. Furthermore, since these prior art attenuators depend for their operation on manipulation of the polarization of the wave being attenuated, care must be taken in their design and construction to ensure that they do not alter the polarization characteristics of the optical signal, thereby affecting the possible dispersion of the signal in its onward transmission. In particular, the attenuator design should ensure that the attenuation does not vary with change in the polarization of the input signal. This means that the polarization dependent loss should be made as small as possible, which is generally accomplished by equalizing the attenuation of the two orthogonal polarizations of the signal.
In U.S. Pat. No. 6,175,667 to F. Wang et al., for “High speed polarization insensitive electro-optical modulator”, there is described a device for use with fiber-optic cables for polarization insensitive amplitude modulation of light comprising a planar electro-optic layer with a reflective conductive mirror electrode on one side and a pair of transparent conductive electrodes on the other, separated along a straight line by a small gap. Light from the input fiber is directed onto the transparent electrodes, through the electro-optic layer and to the mirror from which it is reflected and refocused onto the output fiber. Application of a voltage between either of the transparent electrodes and the reflecting electrode changes the optical path length for half the beam so that when it is refocused on the output fiber, the two halves interfere constructively or destructively, depending on the change in path length and hence on the phase difference between the two halves. The electro-optic layer, depending on the type of effect utilized, changes either its refractive index or its length on application of the voltage in the direction of the propagation, and thus introduces the desired phase shift.
Though the above-mentioned device has the advantage of being, in its basic form, polarization insensitive, it has one major disadvantage. The opto-electric effects used to change the refractive index or the length of the isotropic materials are generally small or insensitive effects, and the electro-optic layer must therefore be thick in order to obtain adequate phase retardation. Alternatively, a comparatively high voltage must be applied between the electrodes, with all of the concomitant problems of generating, controlling and using such a voltage. According to the description in the Wang et al patent, a thick layer also results in a substantial fringing field at the intersection between the split electrodes, which in fact does cause the device to be polarization sensitive. A multiple layer structure is suggested in the Wang et al. patent to overcome the voltage problem and the addition of a quarter wave plate to such a structure to overcome the polarization sensitivity problems, but these additions complicate the structure of the device. Furthermore, as is known in the art, the use of a thick layer, even if divided, generally results in a higher insertion loss for the device.
There thus exists an important need for a simple, electronically controllable variable attenuator, which operates at low control voltages, has low insertion loss, and is insensitive to the polarization of the input optical signal.
The disclosures of each of the publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety.